The answer is, probably.

ZP120s can be used in the S1 system.  With an iOS or android device and the S1 app, you should be able to access the speakers when on the the same local network.  You can also do a factory reset  on the units to clear out all the previous owners setting and start from scratch. You really should factory reset regardless, but you can do a quick test beforehand if you wish.

However, there was a period several years ago where customers were allowed to permanently disable their legacy products for an upgrade discount. The units can’t be used anymore.  If the previous owner did that, then you would not be able to use the units.  That could be what your installer was referring to, but I doubt the previous owner would leave the  system connected and in place of the ZP120s were disabled.  Most likely, the installer is mistaken.

For context, this program was later changed to give customers the upgrade discount without disabling their legacy products.  The previous owner likely already applied for the discounts, but if not, you, as the new owner, can apply for the discounts.

If the above doesn’t help you could grab the full serial numbers off the data plates and contact Sonos.

Dumb me didn’t get the full number or take a photo with my phone while I was at the Sonos and had to crawl back behind the audio shelf a second time as well as several unwanted ladder climbs.

Heh. Been there, done that. Too many times, I’m not smart enough to learn. Although last time I had to go through that, I was able to disconnect the device from where it was, and take it closer to a computer and network jack that I trusted. Made it a lot easier to be on the phone with customer support.